In a high-frequency and large-capacity optical fiber communication system, optical transmission devices to which a waveguide type optical modulator is introduced have been widely used. Among these, an optical modulator, in which LiNbO3 (hereinafter, also referred to as “LN”) having an electro-optic effect is used in a substrate, has been widely used in the high-frequency and large-capacity optical fiber communication system when considering that it is possible to realize optical modulation characteristics in which an optical loss is smaller and a band is wider in comparison to a modulator using a semiconductor-based material such as indium phosphide (InP), silicon (Si), and gallium arsenide (GaAs).
The optical modulator using LN is provided with a Mach-Zehnder optical waveguide, an RF electrode unit that applies a high-frequency signal to the optical waveguide as a modulation signal, and a bias electrode configured to perform various kinds of adjustment so as to maintain modulation characteristics in the optical modulator in a satisfactory manner. Examples of the bias electrode include a bias electrode that applies an electric field to the optical waveguide so as to compensate a bias point variation (so-called a temperature drift phenomenon) that is caused by a temperature variation in an environment, and a bias electrode configured to perform optical phase adjustment.
On the other hand, with regard to a modulation mode in the optical fiber communication system, multi-level modulation such as quadrature phase shift keying (QPSK) and dual polarization-quadrature phase shift keying (DP-QPSK), and a transmission format in which polarization multiplexing is introduced to the multi-level modulation become a mainstream in consideration of a recent tendency of an increase in transmission capacity.
An optical modulator (QPSK modulator) that performs QPSK modulation or an optical modulator (DP-QPSK modulator) that performs DP-QPSK modulation includes a plurality of nest-type Mach-Zehnder optical waveguides, and includes a plurality of high-frequency signal electrodes and a plurality of bias electrodes (for example, refer to PTL 1). Therefore, a device size tends to increase, and thus there is a strong demand for a reduction in size.
In the related art, as a technology for a reduction in size, there is suggested a method capable of reducing a drive voltage even in a short electrode by enhancing a mutual operation between each electrode and an optical waveguide. For example, with regard to each waveguide, there is known a configuration in which a bias electrode is constituted by a comb-like electrode (or a bamboo blind shaped electrode) including a push electrode and a pull electrode to reduce a voltage (bias voltage) to be applied to the bias electrode (for example, refer to PTL 2).
FIG. 13 is a view illustrating an example of a configuration of the DP-QPSK modulator in the related art. For example, a DP-QPSK modulator 1300 is constituted by a nest-type Mach-Zehnder optical waveguide (a bold dotted line in the drawing) and an electrode (hatched portion in the drawing) which are formed on a Z-cut LN substrate 1302. In the optical modulator, light beams from a light source (not illustrated) such as a laser diode are incident from a right direction in the drawing, and modulated light beams are emitted from a left direction in the drawing. For example, the emitted light beams are multiplexed by a space optical system and are incident to an optical fiber that is connected to an optical transmission channel.
The optical waveguide is constituted by an incident waveguide 1304 that receives incident light beams from the right direction in the drawing, an optical branching unit 1306 that branches light beams which propagate through the incident waveguide, and two Mach-Zehnder optical waveguide unit 1310a and 1310b which modulate respective light beams which are branched by the optical branching unit 1306.
The Mach-Zehnder optical waveguide unit 1310a includes an incident waveguide 1312a, an optical branching unit 1314a that branches light beams which propagate through the incident waveguide 1312a, parallel waveguides 1316a and 1318a through which light beams branched in the optical branching unit 1314a propagate, a Y-junction, Y-branch coupler 1320a that multiplexes light beams which propagate through the parallel waveguides 1316a and 1318a, and an emission waveguide 1322a that emits light beams multiplexed in the Y-junction, Y-branch coupler 1320a to the outside. In addition, the Mach-Zehnder optical waveguide unit 1310a includes a Mach-Zehnder optical waveguide 1330a (portion in a rectangle indicated by a dotted line in the drawing) and 1332a (portion in a rectangle indicated by two-dot chain line in the drawing) which are respectively formed at parts of the parallel waveguides 1316a and 1318a. 
A bias electrode 1346a that is constituted by electrodes 1342a and 1344a, and a bias electrode 1352a that is constituted by electrodes 1348a and 1350a are respectively formed on a light emission side (left side in the drawing) of the parallel waveguides 1334a and 1336a of the Mach-Zehnder optical waveguide 1330a, and a light emission side (left side in the drawing) of the parallel waveguides 1338a and 1340a of the Mach-Zehnder optical waveguide 1332a. In addition, a bias electrode 1358a that is constituted by electrodes 1354a and 1356a is formed on a light emission side (left side in the drawing) of the parallel waveguides 1316a and 1318a of the Mach-Zehnder optical waveguide unit 1310a. 
As illustrated in the drawing, a configuration of the Mach-Zehnder optical waveguide unit 1310b is the same as the configuration of the Mach-Zehnder optical waveguide unit 1310a. Accordingly, the optical modulator 1300 includes six bias electrodes indicated by reference numerals 1346a, 1352a, 1358a, 1346b, 1352b, and 1358b. In addition, in the optical modulator 1300, RF electrode, which are respectively constituted by electrodes 1370, 1372, 1374, 1376, 1378, 1380, 1382, 1384, and 1386, are also formed on the eight parallel waveguides 1334a, 1336a, 1338a, 1340a, 1334b, 1336b, 1338b, and 1340b of the four Mach-Zehnder optical waveguides 1330a, 1332a, 1330b, and 1332b. 
Here, the bias electrodes 1346a, 1352a, 1346b, and 1352b are bias electrodes configured to adjust a bias point of an optical modulator that is constituted by the Mach-Zehnder optical waveguides 1330a, 1332a, 1330b, and 1332b, and the bias electrodes 1358a and 1358b are bias electrodes configured to adjust a phase of light beams emitted from the emission waveguides 1322a and 1322b. 
In addition, in the optical modulator 1300, the bias electrodes 1346a, 1352a, 1358a, 1346b, 1352b, and 1358b are configured as a comb-like electrode as illustrated so as to reduce a voltage that is to be applied to the bias electrodes for bias point adjustment or phase adjustment.
However, when using the optical modulator by introducing the optical modulator to a device that is practically used, it is necessary to accurately control a bias voltage in order for the bias point variation not to occur so as to maintain optical transmission characteristics in a satisfactory state by compensating the temperature drift. Accordingly, a low-frequency signal (dither signal) for detection of the bias point variation and a DC voltage (DC bias voltage) for returning the bias point to a predetermined value by compensating the variation are applied to bias electrode configured to compensate a temperature drift.
That is, the variation in the bias point such as the temperature drift is compensated by monitoring an optical signal output from the optical modulator with the photo detector while applying the dither signal to the bias electrode, and by adjusting the DC bias voltage that is applied to the bias electrode so that the intensity of the dither signal included in the optical signal becomes the minimum.
For example, as illustrated in FIG. 13, the photo detector for a control of the DC bias voltage can be realized as photo detectors 1362a and 1362b which are respectively disposed at parts of branched waveguides 1360a and 1360b (that is, a part of a surface of the LN substrate 1302 in which the branched waveguides 1360a and 1360b are formed) which branch a part of emission light beams (modulated light beams) which propagate through the emission waveguides 1322a and 1322b. Outputs of the photo detectors 1362a and 1362b are output to the outside of the optical modulator 1300 through a monitor electrode 1368a that is constituted by electrodes 1364a and 1366a and a monitor electrode 1368b that is constituted by electrodes 1364b and 1366b, respectively.
FIG. 14A, FIG. 14B, and FIG. 14C are views illustrating an example of a configuration that allows a part of light beams propagating through the branched waveguide 1360a to be incident to the photo detector 1362a with a cross-section along the branched waveguide 1360a in a portion of the LN substrate 1302 on which the photo detector 1362a is mounted. In the configuration illustrated in FIG. 14A, the photo detector 1362a is disposed on the branched waveguide 1360a, and the photo detector 1362a is bonded to the branched waveguide 1360a with a transparent resin 1400. In this configuration, light beams (evanescent light beams) emitted from the branched waveguide 1360a are incident to an optical receiver (not illustrated), which is disposed in the vicinity of a lower edge of the photo detector 1362a in the drawing, for reception. In addition, in the configuration illustrated in FIG. 14B, a concavo-convex portion 1402 is provided in a surface of an LN substrate 1302′ on an upper side of a branched waveguide 1360a′ to increase surface roughness so as to scatter light beams, which propagate through the branched waveguide 1360a′, on the surface for radiation of the light beam to the outside of the substrate. The light beams, which are radiated, are incident to the photo detector 1362a for reception. In addition, in the configuration illustrated in FIG. 14C, a groove 1404 is formed in a surface portion of an LN substrate 1302″ in which a branched waveguide 1360a″ is formed, light beams which propagate through the branched waveguide 1360a″ are emitted from a wall surface of the groove, and the emitted light beams are incident to the photo detector 1362a for reception.
With regard to a frequency of the dither signal that is applied to the bias electrode 1358a and the like, particularly, as a frequency that is lower than a high-frequency signal that is applied to the RF electrode, a frequency, which is considered to have no effect on the high-frequency signal, is selected. In addition, in a case where a plurality of bias electrodes are used, a dither signal of a frequency that is different in each bias electrode is used so as to easily determine that the dither signal is applied to which bias electrode.
In this case, the dither signal that is applied to each bias electrode is selected in a range of several kHz to several hundreds of MHz in consideration of a configuration in which the dither signal does not have an effect on an RF signal frequency (typically, several tens of GHz), a configuration in which frequencies are not close to each other, a configuration in which a feedback control can be performed in a necessary speed, and the like.
Generally, in an optical modulator having the above-described configuration in the related art, the temperature drift and the like are compensated in a satisfactory manner, and thus the optical modulator can appropriately operate. However, as described above, in an optical modulator (for example, DP-QPSK modulator) that uses a plurality of bias electrodes, a new problem for a bias voltage control, which does not occur in an optical modulator including one bias electrode, may occur. This problem is specific to the optical modulator that uses the plurality of bias electrodes, and the following phenomenon is observed.                When a dither signal is applied to one bias electrode, an optical characteristic control (phase adjustment or temperature drift compensation) in one or a plurality of other bias electrodes may become unstable. In this case, the unstable phenomenon may be observed even in the one bias electrode in addition to the other bias electrodes in some cases.        The unstable phenomenon may occur not only between bias electrodes which are adjacent or close to each other but also between bias electrodes which are not adjacent to each other or between bias electrodes which are not close to each other.        The unstable phenomenon may occur or may not occur depending on an environmental temperature at the periphery of the optical modulator.        The unstable phenomenon may be solved when changing a frequency of the dither signal to another frequency.        The unstable phenomenon may not occur in a case of applying only a DC voltage to each bias electrode.        
The above-described unstable phenomenon is a phenomenon that cannot be explained with “electrical interference that occurs between electrodes disposed in proximity to each other”, and the cause of the unstable phenomenon has not been known.
The inventors of the invention have made a thorough investigation on the unstable phenomenon in a bias control operation in an optical modulator including a plurality of bias electrodes. As a result, they obtained a finding that the cause of the unstable phenomenon is a surface acoustic wave (SAW) that occurs when a dither signal is applied to the bias electrodes on an LN substrate. That is, when the dither signal is applied to one of the bias electrodes which are formed on the LN substrate, the surface acoustic wave (SAW) occurs on a substrate surface due to a piezoelectric effect of LN that is a substrate raw material, and the surface acoustic wave propagates along the substrate surface and reaches another bias electrode. According to this, the other bias electrode receives the dither signal that is applied to the one bias electrode, and the dither signal that is received give interference to a bias control operation in the other bias electrode and has an adverse effect on the bias control operation.
The surface acoustic wave is an acoustic wave that propagates along the substrate surface, and is reflected and scattered from the substrate surface. The surface acoustic wave also operates on a distant bias electrode that is not close, and intensity or a frequency of the surface acoustic wave varies in accordance with a variation in substrate physical properties (particularly, a propagation velocity of the acoustic wave on the substrate surface, linear expansion of the substrate) due to a temperature variation, and the like. Therefore, the unstable phenomenon occurs between bias electrodes which are not adjacent or close to each other, and the unstable phenomenon occurs depending on the environmental temperature.
In addition, since the comb-like electrode is used as the bias electrode, when the bias electrode is applied with a dither signal of the same frequency as a characteristic frequency at which electro-acoustic conversion efficiency (for example, a ratio of the power of the surface acoustic wave that occurs to the power of the electrical signal that is applied), which represents efficiency at the time of converting the electrical signal applied to the comb-like electrode into the surface acoustic wave, becomes the maximum, the above-described unstable phenomenon becomes significant. In addition, the characteristic frequency is determined by an electrode interval of the comb-like electrode that is the bias electrode.
FIG. 15 is a view illustrating an example of a configuration of the comb-like electrode that can be used in the bias electrode of the optical modulator. A comb-like electrode 1500 illustrated in the drawing is constituted by two electrodes 1502 and 1504. The electrode 1502 includes three electrodes 1510, 1512, and 1514 which extend in parallel to each other in a horizontal direction, and the electrode 1504 includes three electrodes 1520, 1522, and 1524 which extend in parallel to each other in the horizontal direction (hereinafter, electrode portions such as the electrodes 1510, 1512, 1514, 1520, 1522, and 1524 parallel to each other in the comb-like electrode are referred to as “electrodes which constitute the comb-like electrode”). A total of six electrodes 1520, 1510, 1522, 1512, 1524, and 1514 have the same electrode width h and are spaced away from each other by a gap (electrode gap) a. Accordingly, an electrode interval (pitch) p is given by the following Expression (1).[Expression 1]p=a+h  (1)
At this time, a characteristic frequency f0 (that is, a frequency at which the electro-acoustic conversion efficiency becomes the maximum) of the comb-like electrode 1500 is given by the following Expression (2).[Expression 2]f0=v/λ=v/2p=v/2(a+h)  (2)
Here, v is a propagation velocity of the surface acoustic wave on the substrate surface, and λ is a wavelength of the surface acoustic wave. In other words, a comb-like electrode having the electrode interval p has the characteristic frequency f0 expressed by Expression (2). When a voltage signal having the same frequency as the characteristic frequency f0 is applied to the comb-like electrode, the comb-like electrode strongly excites the surface acoustic wave of the same frequency as the characteristic frequency f0. In contrast, when the surface acoustic wave of the same frequency as characteristic frequency f0 is incident to the comb-like electrode having the characteristic frequency f0, an electrical signal having the same frequency as the characteristic frequency f0 is strongly induced to the comb-like electrode. In bias electrodes having the comb-like electrode structure, the induced electrical signal becomes a strong noise signal, and has an adverse effect on a bias control operation.
As is clear from Expression (2), the characteristic frequency f0 can be allowed to vary by changing the electrode width h and/or the electrode gap a to change the electrode interval p.
The propagation velocity v of the surface acoustic wave has a different value depending on a kind of a material that is used in the substrate, a direction of the substrate surface with respect to a molecular arrangement (for example, a crystal orientation) of the material, a propagation direction of the surface acoustic wave, and the like. For example, in a case of using a Y-cut LN substrate as a substrate, in a surface acoustic wave that propagates in a Z direction, the propagation velocity becomes approximately 3500 m/s. In a case of using a 128° Y-cut LN substrate as a substrate, in a surface acoustic wave that propagates in an X direction, the propagation velocity becomes approximately 4000 m/s.
The electrode width h and the electrode gap a are determined in consideration of a transverse field pattern or a filed diameter (typically, approximately 10 μm) of an optical wave that propagates through the optical waveguide.
For example, in a case where the electrode interval is 15 μm, the electrode width is 20 and the velocity of the surface acoustic wave is 3500 m/s, the characteristic frequency f0 becomes approximately 50 MHz. In this case, when a dither signal having a frequency component close to 50 MHz is applied to the bias electrode that is constituted by the comb-like electrode 1500 illustrated in the drawing, a strong surface acoustic wave excites. The surface acoustic wave propagates along the substrate surface toward a direction (an upper and lower direction in the drawing) that is perpendicular to a longitudinal direction of the electrode (for example, the electrode 1520) that constitutes the comb-like electrode 1500, and reaches another bias electrode (comb-like electrode). In the other bias electrode, the surface acoustic wave is converted into an electrical signal due to a piezoelectric effect, and a noise signal of the frequency occurs. The bias control operation is affected by the noise signal.
The degree of the effect on the other bias electrode becomes the strongest in a case where the other bias electrode is disposed at a position to which the surface acoustic wave reaches from the direction perpendicular to the longitudinal direction of an electrode that constitutes the other bias electrode, and the other bias electrode has the same characteristic frequency as a frequency of the surface acoustic wave. In addition, the degree of the effect also depends on a value of electro-acoustic conversion efficiency in the characteristic frequency of the other device, and the greater the efficiency is, the greater the degree is.
The inventors of the invention have devised a new configuration for suppressing or reducing interference between bias electrodes through the surface acoustic wave on the basis of the finding, and have verified that the configuration is effective to suppress the interference (a specific configuration is described in Japanese Patent Application No. 2016-036962).
However, as is the case with the optical modulator 1300 illustrated in FIG. 13, in a case where the photo detectors 1362a and 1362b, which detect the magnitude (detect a bias point) of the dither signal component included in an optical signal (more specifically, emission light propagating through the emission waveguide 1322a) that is modulated by the dither signal applied to the bias electrode 1358a and the like, are provided on the substrate 1302, as a new problem, the surface acoustic wave, which occurs in the bias electrode 1358a, and the like, propagates along the substrate surface of the LN substrate 1302 and reaches a mounting region of the photo detectors 1362a and 1362b. As a result, a variation in power of received light beams may be caused to occur in the photo detectors 1362a and 1362b. 
That is, as illustrated in FIG. 14A, FIG. 14B, and FIG. 14C, the photo detectors 1362a and 1362b are disposed in proximity to an upper portion of the branched waveguides 1360a and 1360b. Therefore, when the surface acoustic wave propagating along the surface of the LN substrate 1302 arrives, a geometric distance between the photo detectors 1362a and 1362b, and the branched waveguides 1360a and 1360b slightly varies. According to this, the quantity of light beams (the quantity of received light beams), which are emitted from the branched waveguides 1360a and 1360b and are received by the photo detectors 1362a and 1362b, may vary due to the slight variation in the geometric distance.
In addition, when the surface acoustic wave acts on the optical waveguide, a state of propagating light beams varies by a photo elasticity effect (a phenomenon in which a refractive index varies when a pressure is applied) and the like, and the amount of light beams received by the photo detectors 1362a and 1362b may vary.
The variation in the quantity of received light beams includes a frequency component (that is, a frequency component of a dither signal that is applied to the bias electrode 1358a and the like) of various propagating surface acoustic waves. Accordingly, a detection error for the intensity of the dither signal component, which is included in emission light beams (modulated light beams) which propagate through the emission waveguide 1322a, is caused to occur in the photo detectors 1362a and 1362b. As a result, the unstable phenomenon occurs in the bias voltage control operation (also referred to as “bias control operation”) that is performed by using the photo detectors 1362a and 1362b, and a transmission quality of the light beams, which are modulated by the optical modulator 1300, deteriorates.
In addition, in a photo detector that is formed on the same substrate as a bias electrode, the detection error for the dither signal component, which is caused by the surface acoustic wave, may occur in an optical modulator that includes a plurality of bias electrodes and can generate a lot of surface acoustic waves as illustrated in FIG. 13, or may occur in an optical modulator including a single bias electrode. Accordingly, it is necessary to find a solution for reducing the detection error.